Digital systems aim to reduce breast imaging radiation dose

Because modern digital detectors can be tuned to be more efficient than screen-film systems, they automatically lower the radiation dose to the patient while maintaining the image quality radiologists crave. Photon-counting, the method used in Sectra's detector, for example, minimizes patient exposure to radiation. In a 2006 analysis, the British National Health Service Breast Screening Programme reported that the Sectra MicroDose Mammography system achieved significant dose savings over screen-film mammography. The average mean glandular dose of the MicroDose System in this analysis was 0.57 mGy, a level that was 75% lower than average dose levels in film imaging systems. A Swedish study1 confirms that clinical performance in terms of cancer detections rate and recall rate can be improved, while decreasing the radiation dose. "The secret to the dose reduction possible with Sectra MicroDose Mammography is a high detective quantum efficiency (DQE) and contrast transfer, which basically translates to very low noise in the system and absorption and use of all incident x-rays with high efficiency," said Prof. Mats Danielsson, a professor of physics at the Royal Institute of Technology in Stockholm and cofounder of Sectra Mamea. "The photon-counting system, in fact, rejects all electronic noise that would otherwise blur the image." Manufacturers of digital mammography systems are seeking to reduce radiation dose to patients by other means as well. One approach involves changing the way in which x-rays are generated. By going to a higher kV, the average x-ray energy will be higher, which means it will penetrate the breast and hit the detector more efficiently, said Dr. Andrew Smith, principal scientist for Hologic of Bedford, Massachusetts. "For the same amount of signal hitting the detector, there is less signal being absorbed in the breast," Smith said. "That is something that could not be done with screen-film imaging because screen-film systems must operate in a very limited kiloelectron volt range to provide a high-quality image. But digital detectors are capable of imaging over a wide range of exposures and incident kV energies. So changing x-ray generation is the strategy by which people are further reducing dose." The focus of change is on composition of the x-ray tube and x-ray filters. The gold standard for analog imaging has been a molybdenum x-ray tube and a molybdenum x-ray filter. However, the x-rays emitted from a tungsten tube have higher energy. "Tungsten generates higher energy, which penetrates the breast better, so less radiation is absorbed in the breast and more goes to the imaging plate itself," said Jonny Eser, mammography product manager in the U.S. for Siemens. "Because of the higher current used to excite the electrons, there is better penetration-particularly in dense breasts-and a reduction in the patient dose." The most common molybdenum form of x-ray tube operates at approximately 35 kV constant potential and produces peaks of radiation at 18 keV and 20 keV. That is the narrow x-ray spectrum of a molybdenum anode target and filter. X-ray tubes with tungsten targets operate at 30 kV, and they generate a large proportion of characteristic radiation at 8 keV and 10 keV. In combination with a rhodium filter, both molybdenum and tungsten x-ray tubes improve the depiction of glandular tissue, pectoral muscle, skin, and subcutaneous tissue for all types of breasts. The tungsten/rhodium combination provides a better depiction of glandular tissue in dense breasts, however, according to various studies. Siemens Medical Systems has used tungsten tube technology in its mammography units since it first released the Mammomat 3000 Nova analog product line. In phantom studies2,3 of its amorphous selenium full-field digital Mammomat Novation system, investigators have shown that tungsten/rhodium preserves image quality while reducing radiation dose. Dose reductions ranged from 28% for an average 4-cm breast to 66% for a thick, fatty breast. Signal-to-noise ratios in